Battery Charger

Solar Battery Charger Circuits

This is the most simple and affordable solar battery charger that the hobbyist can make. It has a few drawbacks over other similar controls, but offers numerous advantages. It is intended for charging lead-acid batteries, but may also be used for charging any battery at a constant voltage. Voltage output is adjustable.Advantages & Disadvantages of this solar charger
+ Simple, small & inexpensive
+ Uses commonly available components
+ Adjustable voltage
+ ZERO battery discharge when sun is not shining
– High drop-out voltage—may be marginal for 6V application
– Current limited to 1.5A
– No LED indicators—no bells or whistles
Solar battery charger specifications
Solar panel rating: 20W (12V) or 10W (6V)
Output voltage range: 5 to 14V (adjustable) (may be reduced further by shorting R2)
Max power dissipation: 10W (includes power dissipation of D1)
Typical dropout voltage: 2 to 2.75V (depending upon load current)
Maximum current: 1.5A (internally limits at about 2.2A)
Voltage regulation: ±100mV (due to regulation of series rectifier)
Battery discharge: 0mA (this control will not discharge the battery when the sun doesn’t shine)
Solar battery charger schematic
6V Applicaton
Output Voltage: Set for 7V
Input voltage:
Battery discharged (6V): 8.75V Min @ 1.5A (this is a little high for panels that are characterized for 6V applications)
Battery charged (7V): 9V Min @ 10mA (e.g.)
12V Application
Output Voltage: Set for 14V
Input voltage:
Battery discharged (12V): 14.75V Min @ 1.5A (Available from solar panel characterized for 12V operation)
Battery charged: (14V): 16V Min
Minimum Head Voltage
This is also referred to “drop-out voltage.” The input voltage must exceed the output voltage by about 2.75V @ 1.5A. Fortunately, when the battery discharged, the output voltage is lower so the solar panel voltage will also be lower.
When fully charged, the battery voltage will be high, but the current is very low—at this point, the drop-out voltage reduces to about 2V and the open circuit solar panel voltage also comes into play. The schottky rectifier was selected to reduce this head voltage requirement—the voltage drop of the schottky is about 0.5V @ 1.5A or about half that of a typical silicon rectifier.
More advanced controls have a much lower head voltage requirement and will function better under marginal conditions.
Maximum Power Dissipation
The power is limited by the thermal resistances of both the LM317T and the heat sink. To keep the junction temperature below the 125°C Max, the power must be limited to about 10W. If a smaller or less effective heat sink is used, the maximum power dissipation must be de-rated. Fortunately, the LM317 has internal temperature limiting so that if it gets too hot, it shuts down thus protecting itself from damage. Max power comes into effect when charging a 12V battery @ 1.5A: e.g. battery voltage = 12V, solar panel = 18V. P = (18V – 12V) * 1.5A = 9W. So thermally, it is carefully matched to the current rating.
If a solar panel that is characterized for 12V is applied with a 6V battery, the maximum current must be reduced to about 0.7A: e.g. battery voltage = 6V, solar panel voltage = 18V. P = (18V – 6V) * 0.7A = 9.6W. In this case,

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